The vestibulo-collic reflex resists disturbances of the head by using semicircular canal and otolith signals to contract neck muscles that oppose head rotation and linear motion. The behavior and neural mechanisms are unknown for vestibulo-collic responses to linear motion. The proposed research has four aims for characterizing the head- stabilizing neck muscle activity produced by vestibular otolith responses to linear motion and exploring the underlying neural mechanisms in behaving squirrel monkeys. 1. Electromyographic activity of the linear vestibulo-collic reflex: We will record electromyographic activity of the major neck muscles in alert squirrel monkeys during the vestibulo-collic reflex elicited by sinusoidal linear motion across a range of frequencies. Many directions of motion will be tested to explore spatial organization of the reflex responses. We will compare the linear vestibulo-collic reflexes to the rotational vestibulo-collic reflexes elicited by the stimuli we have used in past studies. Velocity step and multi-frequency stimuli will be used to assess latency and predictive responses to linear motion and rotation. 2. Modulation of the linear vestibulo-collic reflex: We will record electromyographic activity of the linear vestibulo-collic reflex in different body orientations with respect to gravity to test for modulation of the reflex by static signals and explore the linear addition of vestibular signals to the neck muscles. We will explore paradigms for modulating vestibulo-collic responses with other sensory inputs. 3. Modeling of the linear vestibulo-collic reflex: We will generate computer models of the neuronal signals of the vestibulo-collic reflex, including the linear vestibulo-collic reflex. These models will be based on our models of vestibulo-ocular reflex organization and will test the ability of a realistic model representation of the three neuron vestibulo-collic reflex pathway to produce the responses we observe during rotational and linear vestibular stimulation under aims 1 and 2. Model predictions of neuron responses will be compared to vestibulo- cervical neuron activity recorded under aim 4. 4. Activity of vestibulo-cervical neurons during linear vestibulo-collic reflex: We will study the directionality of identified vestibulo- cervical neurons that are activated by linear motion and test their connections to specific neck muscles. Neck muscle connectivity will be compared to vestibulo-cervical neuron directionality. Vestibulo-cervical neuron activity will be recorded during linear vestibulo-collic reflex modulation or adaptation to search for possible neural mechanisms of alteration of reflex activity. This research is intended to expand our knowledge of sensory-motor neural systems. The results may be relevant to neurological disease and injury such as that underlying torticollis and balance disorders.